This invention relates to fuel cells in general and a system and method for regulating the temperature of a fuel cell to enable operation in low temperature environments.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. A typical fuel cell consists of a fuel electrode (anode) and an oxidant electrode (cathode), separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load (such as an electronic circuit) by an external circuit conductor. In the circuit conductor, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H+) in acid electrolytes, or the hydroxyl ion (OHxe2x88x92) in alkaline electrolytes. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity in the presence of suitable catalysts and because of its high energy density. Similarly, at the fuel cell cathode the most common oxidant is gaseous oxygen, which is readily and economically available from the air for fuel cells used in terrestrial applications.
The ionic conductivity of the electrolyte is a critical parameter that determines the efficiency and operating condition of a fuel cell. In the case of solid polymer electrolyte membrane (PEM) fuel cells, the ionic conductivity of the electrolyte membrane is dependent on the hydration level of the membrane as water molecules are involved in the transport of hydrogen ions across the electrolyte. Typically, fuel cells operate well in the fully hydrated, essentially water-saturated conditions and at room temperature. When the temperature of the cell and in turn, the temperature of the electrolyte membrane drops significantly below room temperature, the performance of the fuel cell deteriorates. This dependency on water content of electrolyte inhibits operation of fuel cells at temperatures below freezing. At these low temperatures the ion mobility of the electrolyte is severely impaired and hence the output of the fuel cell system drops. Therefore, for practical operation of PEM fuel cell systems in cold environments, additional mechanisms are needed to raise and keep the temperature of the fuel cell above the ambient temperature.
Some of the prior art approaches to address this problem involved addition of heaters, catalytic burners, thermal insulation or secondary power sources to the fuel cell system to raise temperature of the fuel cell before startup in low temperature environments. For example, Hamada et al. (U.S. Pat. No. 5,314,762) teaches the use of a catalytic burner in conjunction with a fuel cell to preheat the fuel cell. Others such as Cargnelli et al. (U.S. Pat. No. 5,753,383) describe a fuel cell system incorporating a catalytic burner and a thermoelectric element for operation in low temperature environments. Although the prior art techniques solve the problem of low temperature fuel cell operation, they add complexity and cost while reducing the efficiency of the fuel cell system.